----- Original Message -----
From: Tom DeReggi
To: WISPA General List
Sent: Friday, October 06, 2006 9:09 AM
Subject: [WISPA] WDS PtMP
Background....
In standard WIFI, a principle exists called hidden note, where two CPEs
transmit at the same time and colide because they do not hear each other.
There are three ways to get around that, using WIFI between Client and AP.
1) Polling (Karlnet, Nstream, Proprietary), 2) Use Omnis, so radios can hear
each other if in close proximity, 3) RTS/CTS which effectively solves the
problem at a significant performance degregation. A well know problem with
well known solutions.
mks: Close. It's when two CPE talk at the same time and the AP can't hear
one of them because the other one is louder. This is part of why you should
never build a network using the same size antennas everywhere. And why more
power isn't always better. I try to keep all of my cpe within about 10 dB
of each other.
mks: It can ALSO be where two cpe talk at the same time because they don't
know each other exists. This causes a collision at the ap (it can't
understand either one of them) and after a random backoff time they'll each
try again.
mks: The easy fix to that problem is usually to just add another ap as
you've filled up the one you already have :-).
Issue.....
How does this play our with WDS? AP to AP communication. Sure in PtP its a
non-issue, because there are only two radios involved to complete the link.
But WDS allows PtMP operation.
How does WDS commuication work? Does the Hidden Node problem exist with PtMP
WDS? And if so, is there a way to address it? If so, will it help to make
the CPE's Omnis, so they hear each other?
mks: As I understand it, wds is simply a way for a cpe unit to ALSO act as
an ap. Much like AdHoc mode. Except this time you can put in WDS units
only where needed so that you can go around a corner or two. With AdHoc the
whole network would have to be that way.
My confusion is how WDS/WDS works compared to Station/AP modes.
Example application:
Using 802.11a gear.
5 seperate MTU buildings, spread out within 300 yards of each other.
1 is a Master AP Site, with an Omni, and a second backhaul radio to the
Internet.
4 of the 5 have a direction CPE style antenna pointing to the Master
Antenna.
WDS is used to allow the radios to operate as true transparent bridges, and
to pass per client (5-10 clients per MTU) large packet VLAN traffic.
(Note: There is a reason we did not select Nstreme w/ Polling. It may have
been an incompatibilty with WDS or inabilty to do transparent bridging with
large packets, which standard 802.11 station mode does not support under
protocol. May have been early version of Firmware, not sure if still an
issue)
Why I thought it might be an issue:
Surveys show low noise. However, as more clients have been taken on (2 mbps
average sustained throughput all combined), the Link quality started to
degregate as if the noise floor was rising.
As a tempoirary measure, we switched to 5.2Ghz (indoor only FREQ, which
appeared not to have any detectable noise in standard 802.11 based survey
tools, and was chosen because non-detectable carrier grade gear would not
use those channels). Its hard to believe that the noise floor would be that
high using that freq. So I'm wondering if the noise that I'm hearing is
actually my own CPEs within this project?
The symptom was sparatic higher latency, what typically would happen if
802.11a had frequent retransmissions (native prorocol ARQ).
mks: 5.2 gig is also usable outdoors. I use 5.2 and 5.3 anywhere I can!
Because most others don't :-). But the smart ones do. It's the 5.1 ghz
band that's indoor only.
mks: I think what you are probably seeing is indeed the effects of all mesh
networks that use single radio systems. They all use the same channel and
try to at the same time. That's why I've never liked standard mesh systems.
I don't think they (and feedback such as yours seems to uphold this) will
ever scale to any real use. Sure, put it in an office and feel free to do
email and an occasional print job, but don't do much more than that with
mesh.
I can look at stats to see if there are re-transmissions, but that data is
pointless, as what I want to know is, is the retransmisison because my own
noise or someone elses. Its hard to tell with WiFi, as WiFi doesn't
transmit when its not in use. So testing in the middle of the night, when
clients and users in town are off, may not be meaningful. Its also
possible, that I just have a failing radio card or two, and a totally
different cause.
mks: Well, first, try changing channels around and see if it has any
measureable effect. Next, get ahold of a spectrum analyzer (Bob M. isn't
that far from you, or I can ship mine out to you).
mks: Next, build a proper network! grin. Put in a 5 gig ptmp and/or ptp
system to link up all of the buildings back to the internet. Then use your
2.4 for the inbuilding work. Better yet, use wires to get to the customers
and just build each of them that wants a wlan his own wlan. You'll have a
MUCH happier customer base!
laters,
marlon
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
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